Hyperfine structure due to 33S in its natural abundance of 0.76% has been measured in the electron spin resonance of free radicals produced by x-irradiation of single crystals of N-acetyl-L-cysteine at 77 K. These measurements proved that the radicals produced at 77 K with principal g values of 1.990, 2.006, and 2.214 are monosulfide radicals with the 3p unpaired electron density of 0.70 on the S. They are believed to be negatively charged molecules RCH2S-H or neutral RCH2SH2 radicals in which 90% of the spin density of the captured electron is concentrated in a d-p hybrid orbital on the S. As the temperature is raised to 300 K, these, as well as the carbon-centered radicals produced at the lower temperature, are mostly converted to neutral disulfide radicals RCH2SS like those observed in irradiated cystine.